Add ExperimentalThreads class that exposes two new experimental metrics
for JVM thread deadlock detection:

- jvm.thread.deadlock.count: Number of platform threads in deadlock
  waiting to acquire object monitors or ownable synchronizers.
  Uses ThreadMXBean.findDeadlockedThreads() which detects deadlocks
  involving both object monitor locks and ownable synchronizers
  (e.g. ReentrantLock).

- jvm.thread.monitor_deadlock.count: Number of platform threads in
  deadlock waiting to acquire object monitors only.
  Uses ThreadMXBean.findMonitorDeadlockedThreads() which detects
  deadlocks involving only object monitor locks (synchronized blocks).

These metrics are equivalent to Prometheus client_java's
jvm_threads_deadlocked and jvm_threads_deadlocked_monitor gauges.
They cannot be implemented via JMX YAML rules because deadlock detection
requires invoking MBean operations (findDeadlockedThreads,
findMonitorDeadlockedThreads), not reading MBean attributes.

The metrics are registered as experimental and are emitted only when
experimental telemetry is enabled via:
  -Dotel.instrumentation.runtime-telemetry.emit-experimental-telemetry=true

In Prometheus format they will appear as:
  jvm_thread_deadlock_count
  jvm_thread_monitor_deadlock_count